The present invention relates to refrigeration systems and more particularly relates to heat exchanger assemblies for refrigeration systems.
Typically, vapor compression refrigeration systems include a compressor, refrigerant flow control devices, and a heat exchanger assembly with at least a condenser and an evaporator. Liquid refrigerant in the evaporator is evaporated by absorbing heat from a medium which is thereby cooled. The evaporator is connected to the compressor so that the gaseous refrigerant from the evaporator flows to the compressor which increases the temperature and pressure of this gaseous refrigerant. The condenser receives the hot gaseous refrigerant from the compressor and cools the refrigerant so that it changes state to a liquid refrigerant which flows, through a refrigerant flow control device, back to the evaporator to begin another heat transfer cycle.
In order to improve the efficiency of a vapor compression refrigeration system, the condenser may include a subcooler for reducing the temperature of the liquid refrigerant flowing to the evaporator. For example, U.S. Pat. No. 3,365,900 to Clark, et al. shows a "sensible subcooler" wherein the temperature of liquid refrigerant in the subcooler is reduced below the condenser saturation temperature by circulating cooling water through a tube bundle in the subcooler. No refrigerant is flashed in the subcooler, thus sensible heat, rather than latent heat, transfer occurs in the subcooler. As shown in the Clark, et al. patent the subcooler is designed so that liquid refrigerant from the condenser flows into the subcooler through an opening in a top part of the subcooler. Liquid refrigerant flows out of the subcooler, through an opening in a bottom part of the subcooler, to a refrigerant metering box for the evaporator.
It is not always practical to improve the efficiency of a vapor compression refrigeration system by using a sensible subcooler. For example, if a low pressure refrigerant, such as R-11, is used in the refrigeration system then a sensible subcooler may not be practical because of the large pressure drop that is required to force the condensed liquid refrigerant through the subcooler. Other kinds of subcoolers may be useful in this type of situation. For example, the condenser of the refrigeration system may include a "flash subcooler" wherein liquid refrigerant from the condenser flows to a subcooler wherein a portion of the liquid refrigerant is flashed thereby absorbing heat from the remaining liquid refrigerant in the subcooler. In this context, and as generally used throughout this patent specification, the term "flash subcooler" is somewhat a misnomer because liquid refrigerant is not subcooled to a temperature below the condenser saturation temperature. Instead, there is a lower pressure in the subcooler which yields a subcooler refrigerant saturation temperature which is less than the condenser refrigerant saturation temperature. However, this provides the same refrigeration cycle benefits as a true subcooler wherein liquid refrigerant is subcooled to a temperature below the condenser saturation temperature.
In a flash subcooler, it is necessary to recycle or otherwise despose of the flashed refrigerant in the subcooler. One way of disposing of the flashed refrigerant is to direct the flashed refrigerant from the subcooler back to the compressor for recycling through the condenser. In order to achieve this flow, the subcooler is connected to a second compressor, or to an intermediate pressure stage of a multi-stage compressor if the refrigeration system utilizes a multi-stage compressor, to provide a pressure difference for extracting the gaseous refrigerant from the subcooler. Alternatively, flashed refrigerant in a subcooler may be disposed of by recondensing the flashed refrigerant directly in the subcooler. This is advantageous because it eliminates the need for extracting flashed refrigerant from the subcooler. Also, the subcooler is adaptable for retrofit to vapor compression refrigeration systems having only one, single stage compressor. U.S. Pat. Nos. 4,207,749 and 4,142,381 show vapor compression refrigeration systems utilizing flash subcoolers wherein flashed refrigerant is recondensed directly in the subcooler.
Heat exchanger assemblies for refrigeration systems having condensers with subcoolers as described above, may comprise separate tube and shell constructions, that is, separate vessels, for the evaporator, condenser, and subcooler. However, this type of construction is relatively complex and costly because of valving and piping interconnections between each vessel and because of the separate shell and fluid flow circuits which are part of each vessel. Therefore, in order to reduce costs and complexity, an evaporator and condenser are often constructed as sections of a single vessel with a plate inside the vessel separating the evaporator from the condenser. Sometimes, a subcooler is built in as part of the condenser section of the vessel. For example, U.S. Pat. No. 3,365,900 to Clark, et al., shows such a single vessel construction with a sensible subcooler. In general, a single vessel construction is more reliable than a multiple vessel construction because of the reduction in complexity of the heat exchanger assembly.
A single vessel construction for a heat exchanger assembly incorporating a flash subcooler wherein flashed refrigerant is recondensed in the subcooler is especially desirable because of the versatility of a refrigeration system utilizing such a heat exchanger assembly. As discussed previously, even refrigeration systems having only one, single stage compressor may use such a heat exchanger assembly. Also, a refrigeration system utilizing such a single vessel heat exchanger assembly may be used in a wider range of applications if the heat exchanger assembly may be easily adapted to a variety of design point or full load operating conditions.